There seems to have been discussion of I2C on the arduino before, including a link here:http://wiring.org.co/reference/libraries/Wire/index.html

A library in the wiring language for communicating over I2C, mind I'm a newbie with the arduino so I don't know what it would take to make this work. I2C certainly seems like something that shouldn't need a separate port, my recollection is that it's designed to be easily interfaced with.